Calling JAVA class member from Native C/C++ code

Question

I'm writing an OpenGL C/C++ application which i'm porting to Android through Android NDK, JNI support. I'm having difficulties executing code from JAVA callback signaled from native.

Here is the code:

class OpenGLSurfaceView extends GLSurfaceView 
{
…
     public OpenGLSurfaceView(Context context, int deviceWidth, int deviceHeight) 
    {
        super(context);
        nativeObj = new NativeLib();
        mRenderer = new OpenGLRenderer(context, nativeObj, deviceWidth, deviceHeight);
        setRenderer(mRenderer);
        setRenderMode(RENDERMODE_WHEN_DIRTY);
    }
…
    private void CallBack()
    {
        // force redraw
        requestRender();
    }
}


class OpenGLRenderer implements GLSurfaceView.Renderer 
{
    …
public void onSurfaceCreated(GL10 gl, EGLConfig config) 
    {
        nativeObj.init(…);
        nativeObj.cachejavaobject(JNIEnv *env, jobject obj); // for caching obj on native side
    }

    public void onSurfaceChanged(GL10 gl, int w, int h) 
    {
    }

    public void onDrawFrame(GL10 gl) 
    {
        nativeObj.draw(…);
    }
}

And in native code i have a function texturesLoaded() that is signaled when some textures are loaded completely on another thread and i need to force a refresh from nativeLib.draw(…) on the JAVA side. Here is how i do it :

I cache the JavaVM, jClass, jMethodID on JNI_OnLoad, and gJObjectCached

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)
{
gJVM = jvm;  // cache the JavaVM pointer

LogNativeToAndroidExt("JNILOAD!!!!!!!!!!");
JNIEnv *env;
int status = gJVM->GetEnv((void **)&env, JNI_VERSION_1_6);
if(status < 0)
{
    LogNativeToAndroidExt("Failed to get JNI environment, assuming native thread");
    status = gJVM->AttachCurrentThread(&env, NULL);
    if(status < 0)
    {
        LogNativeToAndroidExt("callback_handler: failed to attach current thread");
        return JNI_ERR;
    }
}

gJClass = env->FindClass("com/android/newlineactivity/NewLineGLSurfaceView");
if(gJClass == NULL)
{
    LogNativeToAndroidExt("Can't find Java class.");
    return JNI_ERR;
}

gJMethodID = env->GetMethodID(gJClass, "callback", "()V");
if(gJMethodID == NULL)
{
    LogNativeToAndroidExt("Can't find Java method void callback()");
    return JNI_ERR;
}

return JNI_VERSION_1_6;

}

JNIEXPORT void Java_com_android_OpenGL_NativeLib_cachejavaobject(JNIEnv* env, jobject obj)
{
    // cache the java object
    gJObjectCached = obj;
...
}

and then on texturesLoaded() i do :

void texturesLoaded()
{
    // Cannot share a JNIEnv between threads. You should share the JavaVM, and use JavaVM->GetEnv to discover the thread's JNIEnv
    JNIEnv *env = NULL;
    int status = gJVM->GetEnv((void **)&env, JNI_VERSION_1_6);
    if(status < 0)
    {
        LogNativeToAndroidExt("callback_handler: failed to get JNI environment, assuming native thread");
        status = gJVM->AttachCurrentThread(&env, NULL);
        if(status < 0)
        {
            LogNativeToAndroidExt("callback_handler: failed to attach current thread");
            return;
        }
    }

    LogNativeToAndroidExt("Calling JAVA method from NATIVE C/C++");
    env->CallVoidMethod(gJObjectCached, gJMethodID);
    LogNativeToAndroidExt("DONE!!!");
}

Result… from native side i get the class, i get the method, method gets called, but when it reaches/calls requestRender() inside(or trying to access any other member method of GLSurfaceView it crashes!)

I cannot try with CallStaticVoidMethod(gJClass, gjMethodID); because then i don't have access to requestRender();

Any ideas or opinions, maybe i'm doing something wrong here.

Thanks

Answer 1

You need to create global references to the class/object that you stash away. The references you're saving are local references, which can't be shared across threads and disappear when the runtime cleans up the JNI local reference stack.

Check out the Sun/Oracle documentation for global and local references, and check out JNI methods JNIEnv::NewGlobalRef and JNIEnv::DeleteGlobalRef.

gJClass = env->NewGlobalRef(env->FindClass( ... ));

gJObjectCached = env->NewGlobalRef(obj);

(Edit: Turns out you don't need global references for method IDs.)